This invention relates to apparatus for thermal processing of semiconductor wafers and, more particularly, to a novel heater assembly employing a blackbody radiation source with a constant planar energy flux for thermal treatment of semiconductor wafers in vacuum.
In the semiconductor industry, semiconductor wafers are frequently subjected to thermal treatment as a part of the process of fabricating both discrete devices and integrated circuits. In the course of processing, the crystalline lattice of the semiconductor material, typically silicon, may be damaged. For example, during ion implantation, the incident energetic ions will break covalent bonds between silicon atoms in the crystalline lattice. The defects in the crystalline lattice are eliminated by thermal treatment, or annealing, of the wafers at a sufficiently high temperature for a sufficiently long time. The thermal treatment that produces annealing of crystal damage also serves to activate the dopant species in the silicon; i.e., the dopant atoms such as boron, phosphorous or arsenic assume substitutional or near-substitutional positions in the crystalline lattice so they may serve as sources of charged carriers.
In the production of thin films of semiconductor materials, it is desirable to increase the grain size of polycrystalline material or to convert amorphous silicon into an epitaxial silicon layer. The application of thermal energy in an appropriate manner may be used to accomplish these objectives.
Phosphosilicate glass (PSG) has been widely used as an insulating layer between conductive elements and as a passivating layer in semiconductor devices. Chemical vapor deposited PSG is relatively nonuniform over the surface area of the device and exhibits poor step coverage. Therefore, PSG layers have been thermally treated to produce plastic flow and result in uniform thickness and tapered step coverage. Pending application Ser. No. 412,455, filed Aug. 27, 1982, and assigned to the assignee of the present application, discloses a method for thermal treatment of PSG layers.
The conventional technique for thermally treating semiconductor materials is furnace annealing. The wafers are treated in the furnace at temperatures of about 900.degree. C. to 1100.degree. C. for times on the order of one-half hour. Annealing under such conditions is generally satisfactory, especially for lower dose implants on the order of 10.sup.10 to 10.sup.14 per cm.sup.2. Activation is virtually always satisfactory for such implants. However, uniformity of dopant distribution is often not obtained, since the time and temperature characteristics are not identical for all wafers in a batch. Also, the annealing of wafers at such temperatures for significant lengths of time produces undesirable spreading or redistribution of the dopant, both laterally and vertically. This is especially undesirable for high dose implants on the order of 10.sup.15 to 2.times.10.sup.16 per cm.sup.2, such as are used in fabricating high density MOS devices. Spreading also makes shallow junction and/or VLSI devices difficult if not impossible to fabricate. Conventional furnace annealing is time-consuming and is not particularly energy efficient.
Various techniques have been proposed for producing the rapid thermal treatment of semiconductor materials. It has been found that fast annealing is possible with laser beams and electron beams. However, both require mechanical, electro-optical or electromechanical beam scanning means. In addition, laser beams are highly inefficient and, because of the extremely rapid heating of the wafer, can cause cracking or peeling of oxide layers. Electron beams are relatively energy efficient, but produce neutral traps near insulator-semiconductor junctions which can result in charging effects in operating devices over time. Flash lamps and arc lamps have also been used to thermally treat semiconductor materials. This approach has the advantage that it heats the whole wafer at the same time and eliminates thermal nonuniformities. However, the process is not energy efficient and may require complex optical elements.
A thermal processing system employing a blackbody radiator having a constant planar energy flux characteristic is disclosed in pending application Ser. No. 262,838, filed May 12, 1981, and assigned to the assignee of the present application. A semiconductor wafer is positioned on a platen which is then rotated into parallel alignment with a fixed position blackbody source. Between processing of successive wafers, a shutter is moved in front of the blackbody source to contain radiant energy. The system is capable of annealing crystal damage in semiconductor wafers and of activating impurity dopants in the silicon in times on the order of ten seconds.
The above-described isothermal annealing system has provided highly satisfactory performance and energy efficiency. However, it is desired to further improve the energy efficiency as much as possible. Due to the movement of the platen and wafer relative to the blackbody source, the relative positions of these elements will be subject to certain tolerances and variations. Furthermore, a non-negligible portion of the energy from the blackbody source radiates to the walls of the vacuum chamber and causes heating thereof. Excessive heating of O-rings used to seal the processing chamber can cause reliability problems.
It is an object of the present invention to provide apparatus for energy efficient thermal processing of a semiconductor wafer.
It is another object of the present invention to provide mechanically stable positioning of a semiconductor wafer relative to a thermal source during thermal processing.
It is still another object of the present invention to provide a heater assembly for thermal processing of a semiconductor wafer in a vacuum chamber, wherein the thermal energy is substantially confined to the wafer processing region without requiring a movable shutter.